Is using necrocarnum such an evil act that a character who is otherwise good can qualify for the necrocarnate class solely on the basis of being willing to use necrocarnum? Sort of a the-ends-justify-the-means type situation.

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Aligned Soulmelds: You cannot shape soulmelds with an alignment descriptor that does not match your own.

(Magic of Incarnum pg. 21 and 26, for incarnate and soulborn respectively)

So incarnates and soulborns (the two classes with native access to necrocarnum soulmelds) cannot shape soulmelds the have an alignment descriptor that they lack . Since all necrocarnum soulmelds are evil (pg. 53), that means incarnates and soulborns must be evil to even try to shape a necrocarnum soulmeld. The Necrocarnum Adept feat can allow incarnates and soulborns to shape these soulmelds, but that feat requires being non-good, so good characters cannot take it and characters who have it cannot use it if they become good.

Totemists do not have the aligned soulmelds feature that restricts incarnates and soulborns. On the other hand, they also get no native access to necrocarnum soulmelds.

Then there are people who take Shape Soulmeld. These get very few rules at all, just referencing “the normal meldshaping rules (page 49). Those rules mention the necrocarnum descriptor on page 53, but only go as far as to say that all such soulmelds have the evil descriptor. This implies that, strictly speaking, you can shape necrocarnum soulmelds through the Shape Soulmeld feat even if you are good.

Magic of Incarnum is frustratingly silent on how Shape Soulmeld works for people who are already meldshapers thanks to a class. This leaves a lot of questions unanswered, but relevant today is the fact that it is unclear if a good incarnate or soulborn could use Shape Soulmeld to shape a necrocarnum soulmeld.

In any event, page 53 describes necrocarnum soulmelds thusly:

The necrocarnum descriptor indicates a soulmeld that draws on a vile and corrupted form of incarnum. Such soulmelds also carry the evil descriptor.

And since Magic of Incarnum implements soulmeld–magic transparency on page 52, the rule about evil spells probably applies to evil soulmelds, namely that using one is an evil act, so you wouldn’t stay good terribly long using necrocarnum most likely.

And, in any event, necrocarnate requires that you be evil, at least to start it. A necrocarnate could be redeemed, and at least according to the technical rules in Dungeon Master’s Guide, would keep his powers and could continue talking class levels, but it would be really hard to justify maintaining any not-evil alignment if you keep using necrocarnum. Necrocarnum is one of the most “for teh evulz” options in the whole system, it’s so over the top evil. It literally involves flaying innocent souls into soul-stuff you can manipulate. You basically run on orphans. Even the frankly cartoonish Book of Vile Darkness probably can’t compete with that level of evil for evil’s sake.